1. Field of the Invention
The present invention relates to a semiconductor device and a method for production thereof. More particularly, the present invention relates to a technology to permit miniaturization of semiconductor integrated circuits by providing the substrate with a dopant-rich thin layer buried therein.
2. Description of the Related Art
Silicon integrated circuits owe their high integration level and high speed to miniaturization of fabrication dimensions. However, in the case of MOSFET (metal oxide semiconductor field effect transistor), miniaturization poses a problem with punch-through, which is a phenomenon that current flows from the drain to the source due to a short circuit between them. One way to address this problem was by increasing doping concentration of the region between the source and the drain whose doping type is opposite to that of the source and the drain. This conventional approach, however, is not satisfactory when miniaturization progresses to such an extent that the gate is shorter than 0.1 xcexcm. In this state, doping increases the capacity of the source and drain too much, thereby sacrificing device characteristics.
An ideal method for preventing punch-through, without sacrificing device characteristics, is disclosed in Japanese Patent Laid-open Nos. 95571/1978 and 116775/1986. This method consists in forming a delta doping layer (as thin as several atomic layers with a high dopant concentration) in the position where short-circuiting is liable to occur. The delta doping layer may be formed by causing a doping impurity to be adsorbed to the surface as disclosed in Japanese Patent Laid-open No. 283432/1994. This method, however, is not used for production of MOSFET because it presents difficulties in controlling the amount of adsorption, it needs complex procedure to specify the position of adsorption, and it requires complex equipment which leads to a high production cost.
It is an object of the present invention to provide a method for forming a delta doping layer which prevents punch-through hindering miniaturization of MOSFET without sacrificing the device characteristics in an accurate, stable and inexpensive manner.
According to the present invention, prevention of punch-through is accomplished by introducing dopant atoms into the semiconductor substrate through ion implantation and subsequently causing the dopant atoms to segregate in the surface, with the resulting surface layer functioning as the delta doping layer. Ion implantation offers a technical advantage of permitting one to accurately specify the doping position and accurately control the doping amount. It also offers an economical advantage of requiring no additional investment in production facilities. Incidentally, the term xe2x80x9csegregationxe2x80x9d denotes a phenomenon that impurity atoms distribute themselves in the semiconductor substrate such that their concentration is higher in the surface than in the inside.
It is a well-known fact that heavily doped impurity atoms separate out (or segregate) in the surface of semiconductor. Such segregation has been regarded as undesirable from the standpoint of keeping the doping amount as high as possible. Contrary to this prevailing idea, the present invention makes the best use of surface segregation.
Other and further objects, features and advantages of the invention will appear more fully from the following description.